Emulsion inks are generally used in digital duplicating processes. In those processes, the ink is introduced into a cylinder having a plurality of small holes. The circumferential wall of the cylinder is formed with such holes, and a stencil is wrapped around the cylinder. During the printing process, the ink penetrates through the holes in the cylinder and the selectively formed image openings in the stencil onto the surface of a paper. The paper is held in contact with the cylinder and stencil by means of a platen roller.
Emulsion inks are formulated with an oil phase and an aqueous phase. The oil phase generally contains drying oils. The colorant is dispersed either in the oil phase or in the water phase. Several ink formulations having carbon black colorant dispersed in the aqueous phase are disclosed in U.S. Pat. Nos. 2,839,412, 3,421,910, and 3,823,020. U.S. Pat. No. 5,378,739 discloses an ink formulation having carbon black dispersed in the oil phase.
The fibers of the paper have numerous small pores, which act to break down the two phases in the ink as the ink is absorbed into the paper. The water in the aqueous phase diffuses through the paper leaving behind the carbon black on the surface. The drying oils of the oil phase dry and form a film on the carbon black and immobilize the carbon black on the paper to produce a permanent image.
Certain problems have been experienced in using ink formulations having the colorant dispersed in the aqueous phase, particularly the "bleed through" problem. The water from the water phase that diffuses through the pores of the paper carries the colorant along with it. This "bleed through" results in the colorant being deposited on the back side of the paper. The colorant deposited on the back side becomes visible and the quality of the printed image thus becomes poor.
Inks heretofore known and that have the colorant dispersed in the oil phase also present certain problems. For instance, the ink formulation of the '739 patent presents certain problems. The '739 ink is composed of an oil phase and a water phase with the respective ratios by wt. % thereof being (20 to 40):(80 to 60). The oil phase is composed of a solvent component containing a volatile solvent with an initial boiling point of 150.degree. C. to 210.degree. C., and a non-volatile solvent, with the respective ratios by wt. % thereof being (10 to 30):(90 to 70), a coloring agent, a resin, and a surface active agent, with the amount of the resin in the oil phase being in the range of 2 to 8 wt. % of the total weight of the emulsion. The amount and the volatile nature of the solvents used in the '739 ink may pose an environmental hazard.
U.S. Pat. No. 4,069,179 discloses an emulsion ink suitable for typewriter ribbons, carbon papers, and the like, comprising an aqueous phase containing a solubilized partially hydrolyzed polyvinyl acetate polymer and an oil phase dispersed therein and comprising a colorant, a fatty alcohol having even number of carbon atoms in the range of 12-20, and a fatty acid. This ink is not considered suitable for use in digital duplicators in view of the poor quality of the image produced when this ink is used in digital duplicators.
Certain emulsion inks heretofore known use excessive amounts of oils and surfactants which tend to impair print quality. The surfactants and oils diffuse through the paper horizontally and vertically and make the paper become transparent.
The colorants dispersed in the water phase or in the oil phase of certain ink formulations heretofore known contain agglomerations of the colorants. The agglomeration of colorant particles results in an ink having a mixture of large and small colorant particles. Such inks have been found to give poor duplicator runnabilty due to clogging of the cylinder mesh by the large particles. These inks therefore offer poor image quality, weak color strength, low image density, poor set off and poor image resolution. The agglomeration of the colorant particles, which is due to the surface energies of the particles, has not been prevented due to the fact that the colorants used in these formulations have not been subjected to any coating or other surface treatment that would prevent the agglomeration of the particles.
Attempts have been made in the industry to modify the surface of the colorant particles by encapsulating the particles with a polymer. For instance, U.S. Pat. No. 4,421,660 discloses a method of encapsulation which involves emulsion polymerization of polymerizable monomers for surrounding discrete colorant particles with a polymer matrix. As one of ordinary skill in the art would know, carrying out the emulsion polymerization has certain complexity associated with it; for example, it requires a new investigation of appropriate conditions to induce a polymerization for a desired concentration and material for each specific application to which the encapsulated colorant will be used for.
U.S. Pat. No. 4,264,700 discloses a suspension polymerization method of encapsulation of solid toner particles, wherein the toner particles are suspended in an aqueous phase and the monomer is dissolved in the organic phase. The monomer is emulsified in order to allow migration of the monomer through the aqueous phase and polymerize at active sites on the toner particles. This method also involves certain complexity and requires investigation of the appropriate conditions and materials to accomplish the suspension polymerization and the encapsulation.
U.S. Pat. Nos. 4,439,510 and 4,447,516 disclose a method of encapsulating toner particles useful in electrostatography using interfacial polycondensation techniques. The toner particles, such as magnetite or carbon black, are suspended in a paraffinic solvent containing a stabilizer such as lecithin or polyvinyl alcohol. A reactive compound such as an acid chloride, for instance, terephthaloyl chloride, is also dissolved in the paraffinic solvent. An aqueous solution of a reactive amine, such as diethylene triamine, is added to the paraffinic suspension and stirred. The interfacial polycondensation between the acid chloride and the amine results in the formation of a polyamide coating on the toner particles. This method also involves the complexity of determining the appropriate conditions for forming the suitable encapsulating polymer for each type of particle. Moreover, this method involves the use of corrosive acid chlorides, harmful amines and volatile solvents.
U.S. Pat. No. 4,665,107 discloses a process for modifying colorants suitable for preparing writing fluids, textile coloration fluids, and cosmetic compositions, which comprises modifying the surface of an active ingredient of solid primary particles in the submicron range by encapsulating with polymeric starting materials that are not substantially altered during the process. For instance, the process of preparing a writing fluid containing colorant latex is as follows. An organic solvent such as methylene chloride containing a small portion of 2-amino-2-methyl-1-propanol is mixed with an alkylated vinylpyrrolidone copolymer, and a water insoluble colorant is dispersed into the mixture. A separate water mixture containing water, an anionic surfactant, and a styrene maleic anhydride resin is prepared. The organic solvent mixture is then emulsified into the water mixture. The organic solvent is removed from the emulsion by evaporation to produce a colorant encapsulated latex having about 8% polymer, 2% colorant, and 89% water. The above process does not disclose an encapsulated colorant suitable for digital duplicating and moreover utilizes volatile organic solvents such as methylene chloride which is considered harmful to the environment.
Co-pending and commonly owned U.S. patent applications Ser. No. 08/267,039, filed Jun. 27, 1994, and U. S. Ser. No. 08/447,683, filed May 23, 1995, disclose certain encapsulated pigments suitable for use in printing inks and in magnetic recording media. The pigment is encapsulated by a composition comprising a vegetable oil such as soya oil and optionally an ionomer such as polyethylene-acrylic acid copolymer. The applications, however, are not directed to the preparation of water-in-oil emulsion inks suitable for use in digital duplicators.
From the foregoing, it is clear that there exists a need for water-in-oil emulsion inks that produce high color strength, high image density, good image resolution, low set-off, low "bleed through" excellent runnability, and better environmental stability.
A need therefore exists for a water-in-oil emulsion ink suitable for use in digital duplicators that offers high color strength, high image density, good image resolution, low set-off, low "bleed through", excellent runnability, and better environmental stability.
A need also exists for an encapsulated colorant suitable for the preparation of the water-in-oil emulsion inks of the present invention.